Lens Having a Fixed Focal Length and Constant Overall Length for Auto Focus Applications

ABSTRACT

A lens with a fixed focal length includes five lens-element groups, a first front lens-element group, a second lens-element group, a third lens-element group, a fourth lens-element group, and a fifth back lens-element group is arranged in a stationary manner. Relative to an imaging plane in a lens barrel and both the focusing front group and the focusing back group are movable jointly relative to one another and to the lens-element groups arranged in a stationary manner in order to focus the lens on objects at different object distances.

The embodiments of the invention relate to an interchangeable lens witha fixed focal length. Such lenses are used for photographic imagerecording purposes from analog photography and are likewise used fordigital image recording. To an increasing extent digital cameras nolonger have a mirror which is pivotable into and out of the imagerecording beam path and which deflects the object field to bephotographed into a viewfinder via a prism for focusing purposes and inorder to select the image detail, rather the image selection is carriedout by permanent image recording by means of the image recording sensorand on the basis of an object detail obtained therefrom, which isrepresented on a display on the rear side of the camera, or with the aidof an electronic viewfinder. The focusing of these lenses is carried outautomatically with the aid of electronic autofocus signals andcorresponding driving of the focusing element in the lens. Photographiclenses for producing a good imaging performance usually consist of twoor more lens-element groups, which in turn have individual lens elementsmounted in a stationary manner or in a displaceable manner along theoptical axis. A lens-element group may be displaceable along the opticalaxis for the purpose of focusing the lens at different object distances.This can be for example the lens head, that is to say the frontlens-element group facing the object plane, or the complete lens. Suchan arrangement is also called total focusing. With this type offocusing, however, the structural length of the lens changes duringfocusing, which contains disadvantages with regard to tightness.Therefore, lenses in which a lens element mounted in a displaceablemanner in the lens, the so-called focusing element, may be displacedalong the optical axis. Such an arrangement is also called internalfocusing. It is true that lenses of this design can be focused in wideranges from an infinite object distance through to close range of a fewmeters or even centimeters, that is to say that the object is imagedsharply onto the image recording plane; however, the optical imagingperformance decreases precisely at close range. Optical imageaberrations such as distortion, image field curvature, apertureaberrations, chromatic aberrations and coma increase. Despite focusingat the desired object distance, the image results then often no longersatisfy the requirements in respect of the imaging performance of modernimage recording systems. If a high imaging performance is neverthelessintended to be achieved, complicated and expensive lens designs having ahigh number of different lens elements are required. Alternatively, inorder to increase the imaging performance in the case of lenses withtotal focusing, principally for close range, a second movable lenselement may be provided, a so-called float element, which counteractsthe imaging aberrations, but for the rest has no influence on the focusposition.

In order to realize lenses with variable focal lengths (zoom lenses), atleast two lens elements or lens-element groups which are adjustablerelative to one another on curves are normally provided. The focusing isstill carried out by means of a focus element that is adjustable forfocusing. In the case of such lenses, accordingly, one group from twolens-element groups performs the focal length variation and,independently thereof, a further lens-element group is used forfocusing.

One such lens with a variable focal length is disclosed in US2013/0070124 A1, for example. This lens comprises three movablelens-element groups for focal length and focus variation.

U.S. Pat. No. 8,619,374 B2 discloses an interchangeable lens with avariable focal length. A stationary front lens-element group is followedby an axially adjustable lens-element group for changing the focallength. Two mutually independently adjustable focusing lens-elementgroups are inserted between two further stationary lens-element groups.Imaging aberrations that arise depending on the change in focal lengthare intended to be compensated for with the aid of the two focusinglens-element groups.

The problem addressed by the embodiments of the invention was that ofmaking possible, in the case of interchangeable lenses with a fixedfocal length, a very high constant image quality during focusing atdifferent object distances from infinity to extremely close range atless than 30 cm or with an imaging scale of up to 1:3, wherein the lenselements required for focusing should have a simple construction withlow weight in order to be able to achieve a fast and quiet autofocusdrive having high accelerations. Furthermore, the lenses should besuitable for use on mirrorless recording systems with a short flangefocal distance, have a short back focal distance and at the same timehave a sufficiently large distance between the exit pupil of the lensand the imaging plane. For suitability for modern image sensors, theangle of incidence of light in the imaging plane, as viewed from theperpendicular, should not become too large.

This problem is solved according to the invention by means of theinventive lens. With regard to the solution features specified in theclaims it should be noted that in modern optical design use is usuallymade of automatic correction programs, such as e.g. “Code V” fromOptical Research Associates, which are able to calculate, frompredefined lens-element sequences and refractive power distributions,proposals for functional lens systems having a correction stateoptimized for a specific task. On the basis of targeted alterations ofthe specified parameters by the optical designer, the correction stateautomatically achieved is improved further in each case.

With the features of claim 1, it is possible in this way already toobtain the design data for radii, lens-element thicknesses, distancesbetween lens elements, refractive indices and Abbe numbers of theoptical glasses to be used. When the features specified in the dependentclaims are taken into account, the design parameters can beprogressively improved in a targeted manner.

Exemplary embodiments of the lens according to the invention areillustrated to scale in the drawing; design data can be gathered fromthe tables assigned to respective figures. It is evident to the personskilled in the art, an optical designer, that the design complexity forlenses according to the embodiments of the invention decreases if alower optical imaging performance is accepted in return.

The solution feature of lenses according to the embodiments of theinvention consists in providing in a lens barrel two focusinglens-element groups mounted in a displaceable manner along an opticalaxis with respect to an imaging plane, wherein a focusing front group asviewed from the object side is arranged upstream, and a focusing backgroup as viewed from the object side is arranged downstream, of astationary central group having lens elements and an aperture stop (irisdiaphragm). As a result of the control of the two focusing lens-elementgroups jointly relative to one another and to the other lens-elementgroups arranged in a stationary manner in the lens barrel, the course ofthe image aberrations that is introduced during focusing at differentobject distances is advantageously mutually compensated for. Inaddition, a lens according to the embodiments of the invention comprisesa front lens-element group, which is stationary as viewed from theobject side, and a stationary back lens-element group facing the imagingplane. In this way, a lens with a fixed focal length is realized whichconsists of five lens-element groups, of which three are mounted in astationary manner and two are mounted in a displaceable manner along theoptical axis for focusing purposes.

In contrast to lenses having total focusing with a floating element, inthe case of the focusing according to the embodiments of the invention(also called double focusing hereinafter) the two focusing lens-elementgroups jointly perform the displacement of the focus position forfocusing the object plane onto the imaging plane. The travel of thefocusing groups for focusing from infinity to the near setting (maximummovement distance corresponds to the total travel) is limited by themechanical structural length of the lens and the motor/drive concept ofthe autofocus. In this case, the ratio of the two respective totaltravels to one another can be one or else not equal to one. It resultsfrom design-dictated spatial limitations and can be varied in order tooptimize the aberrations. According to the embodiments of the invention,in this case the front lens-element group and the back lens-elementgroup have a negative refractive power, or the front lens-element groupand the back lens-element group have a positive refractive power. In analternative embodiment, the front lens-element group has a positiverefractive power and the back lens-element group has a negativerefractive power or the front lens-element group has a negativerefractive power and the back lens-element group has a positiverefractive power.

In one particular embodiment of the lenses, both the focusing frontgroup G2 and the focusing back group G4 have a positive refractive powerand move away from the imaging plane IM during focusing from infinity tothe near setting.

In an alternative embodiment of the lenses, the focusing front group G2has a positive refractive power and the focusing back group G4 has anegative refractive power, wherein the focusing front group moves awayfrom the imaging plane IM during focusing from infinity to the nearsetting and the focusing back group moves toward the imaging planeduring focusing from infinity to the near setting.

In a further embodiment of the lenses, both the focusing front group G2and the focusing back group G4 move toward the imaging plane IM duringfocusing from infinity to the near setting, wherein both focusing groupshave a negative refractive power.

In a further embodiment of the lenses according to the embodiments ofthe invention, the focusing front group G2 has a negative refractivepower and the focusing back group G4 has a positive refractive power,wherein the focusing front group moves toward the imaging plane IMduring focusing from infinity to the near setting and the focusing backgroup moves away from the imaging plane during focusing from infinity tothe near setting.

In the case of lenses for the full-frame format (image circle diameter43.3 mm) with focal lengths e.g. between 13 mm and 65 mm, it has beenfound to be advantageous to use more lens elements having a negativerefractive power than having a positive refractive power in the frontlens-element group or to embody said lens elements as having overallnegative refractive power. In the case of full-frame format lenses withfocal lengths, e.g. between 55 mm and 185 mm, advantageously more lenselements having a positive refractive power than having a negativerefractive power are used in the front lens-element group, wherein thefront lens-element group has a positive total refractive power.

In the case of lenses according to the embodiments of the inventionhaving central groups with a positive total refractive power, afavorable distribution of the refractive powers in the back assembly andcompliance with design stipulations arising for example from thepredefined maximum external dimensions of the lens, the predefinedmaximum aperture and lens-element diameters and from the limitingdimensions of a camera bayonet, in particular the free internal diameterthereof are ensured.

Exemplary embodiments of such lenses are illustrated in FIGS. 3 to 9 ofthe drawing and are described in even greater detail with reference totables with focal length indications [f′] for the five lens-elementgroups and refractive power values [Dpt.].

In this way, it is possible preferably to realize, e.g. a 24 mm, 50 mmor 90 mm lens having a maximum aperture number of 2.0, or else a 50 mmlens having a maximum aperture number of 1.4, wherein the focal lengthindications in this case relate to the full-frame format (image circlediameter 43.3 mm). For the person skilled in the art it is possible,whilst complying with the parameters specified in the patent claims, torealize lenses having a smaller (down to e.g. 0.9) or larger (e.g. 4.0)maximum aperture number. With a smaller maximum aperture number thedesign complexity increases, which has disadvantageous effects on thelens volume, the number of lens elements required and the lens-elementdiameters if the optical imaging performance (aberrations) is intendednot to deteriorate to an excessively great extent. With a larger maximumaperture number, by contrast, the design complexity usually decreaseswith the imaging performance remaining the same or being poorer.

A scaling of the geometric data of the lens to other image formats ispossible with the respective maximum aperture number being maintainedand brings about a corresponding scaling of the focal length. Lensesrealized in this way, with design features otherwise corresponding withrespect to the embodiments of the invention, are likewise the subjectmatter of the invention.

In one particular embodiment of the lens according to the invention, theratio f1/f of the focal lengths f1 of the front lens-element group and fof the entire lens lies between −40 and 30, the ratio f2/f of the focallengths f2 of the focusing front group and f of the entire lens liesbetween −10 and 20, the ratio f3/f of the focal lengths f3 of thecentral group and f of the entire lens lies between −30 and 40, theratio f4/f of the focal lengths f4 of the focusing back group and f ofthe entire lens lies between −10 and 20, and the ratio f5/f of the focallengths f5 of the back lens-element group and f of the entire lens liesbetween −40 and 30.

In a first step of optimizing the ratio f1/f, limitation to a range ofbetween −2.0 and −0.8 or between 0.4 and 5.0 has proved to be expedient.In order to avoid an excessive sensitivity with regard to mountingtolerances to be complied with, it is particularly advantageous to limitthe ratio downwardly in terms of value (small absolute values), and inorder to achieve a structural size that is as compact as possible, theratio f1/f should be limited upwardly in terms of value (large absolutevalues). An embodiment of the invention that is optimized with regard tomounting tolerances and structural size therefore has a ratio f1/f of−1.7 and −1.0, or values therebetween, or of 0.5 and 2.1, or valuestherebetween.

In one particular embodiment of the lens according to the invention, areduction of the focusing travel of the focusing front group and of thestructural length of the lens can be achieved by means of a limitationof the ratio f2/f to a range of between −1.0 and −0.3 or between 1.0 and10.0. In general, small focusing travels are advantageous for fastfocusing, but they also increase the tolerance sensitivity. On the otherhand, an excessively large focusing travel firstly increases thestructural length and secondly requires powerful and fast motorizeddrives having a high energy consumption for focusing. The limitation ofthe ratio f2/f to −0.7 and −0.4 or 1.3 and 5.6, or a range of betweenone of the two abovementioned ranges, is therefore particularlyadvantageous for a double focusing according to the embodiments of theinvention.

An advantageous optimization carried out in the same way for thefocusing back group is manifested in the case of a limitation of theratio f4/f to a range of between −5.0 and 5.0, wherein a furtheroptimization can be achieved by a limitation −1.9 and −0.8 or to 0.6 and0.9, or a range between −1.9 and −0.8 or between 0.6 and 0.9.

An advantageous configuration of the lens elements of the central groupwith regard to manufacturing tolerances and structural size is achievedby means of a limitation of the ratio f3/f to a range of between −2.0and −0.5 or 0.2 and 5.0, wherein an optimization according to theembodiments of the invention resides in a ratio f3/f in a range ofbetween of −1.2 and −1.0 or 0.4 and 3.4, or in a range of between −1.2and −1.0 or between 0.4 and 3.4.

Large absolute values of the ratio f5/f of the back lens element groupresult in a disadvantageously long structural length; with smallabsolute values, the lens-element group becomes sensitive with regard tomounting tolerances. An advantageous optimization therefore consists ina limitation of the ratio f5/f in a range of between −28.0 and −0.6 orbetween 0.5 and 10.0. Particularly in the case of lenses for mirrorlessphotographic cameras, in which experience shows that there are veryshort distances between the last lens element (located near the imagingplane) in the light direction and the imaging plane, it is advantageousto embody the angle of incidence of the light, as viewed from theperpendicular to the imaging plane, such that it is not too large. Anangle of incidence of the light of less than 35° has proved to beadvantageous. The lenses are thus particularly suitable for camerasystems having a short flange focal distance and a short back focaldistance, e.g. less than 25 mm in each case, relative to the full-frameformat described above. Excessively small absolute values of the ratiof5/f therefore have a disadvantageous effect since they increase theangle of incidence of the light and vignetting thus increases. Anembodiment of the lens that is optimized with regard to this problemarea therefore has values of −21.0 and −0.8 or 0.8 and 5.2, or in eachcase a value in the range therebetween, for the ratio f5/f.

In one development according to the embodiments of the invention, thelens has a ratio of the total focal length f to the image circlediameter in the imaging plane (IM) of between 0.3 and 5. In this way, itis possible to realize lenses with a sufficiently large distance betweenthe exit pupil and the imaging plane, e.g. greater than 40 mm, and witha focal length of between 13 mm and 216.5 mm, relative to the full-frameformat described above.

Lightweight focusing groups are advantageous for a high focusing speed.A ratio of the volume V of respectively the focusing front group andfocusing back group to the image circle diameter in the imaging planeraised to the third power is less than 0.1 (V/Bd³<0.1) and in particularlies below 0.08 (V/Bd³<0.08). Particularly advantageously the respectivefocusing group has a weight of less than 10 g (grams).

The value for the relative volume of 0.08, relative to the full-frameformat, then corresponds to a weight of 23.2 g in the case of alightweight glass, such as, for example, N-PSK53A from Schott, and to aweight of 35.8 g in the case of a heavy glass, such as, for example,N-LASF31A from Schott. In the case of medium-format systems having animage circle diameter 1.5 times larger, lens-element weights of 78.3 gin the case of a lightweight glass and 120.8 g in the case of a heavyglass thus result. In the case of APS systems having an image circlediameter 1.5 times smaller, lens-element weights of 6.9 g in the case ofa lightweight glass and 10.6 g in the case of a heavy glass thus result.

A low weight is advantageous for the focusing speed and has anadvantageous effect on motor and noise concepts.

In one particular embodiment of the lens according to the invention, thefront lens-element group consists of four lens elements, wherein thefirst lens element, the second lens element and the third lens elementhave a positive refractive power and the fourth lens element has anegative refractive power wherein the third lens element and the fourthlens element are combined to form a lens-element doublet having anegative total refractive power. One exemplary embodiment comprising afront lens-element group according to the invention is illustrated aslens 1:2 90 mm and 1:2 75 in FIGS. 1 and 2 of the drawing and isdescribed in even greater detail with reference to the associated tableswith focal length indications [f′] and refractive power values [Dpt.].

In an alternative lens, the front lens-element group consists of fourlens elements, wherein the first lens element and the second lenselement have a positive refractive power, the third lens element has anegative refractive power and the fourth lens element has a positiverefractive power, wherein either the third lens element and the fourthlens element are combined to form a lens-element doublet having anegative total refractive power, or the second lens element, the thirdlens element and the fourth lens element are combined to form a cementedelement having a positive total refractive power. The drawing containsin this respect exemplary embodiments which are elucidated in morespecific detail in FIG. 6 and FIG. 7 and which are described in evenmore specific detail as 1:2 90 mm lenses with reference to theassociated tables with focal length indications [f′] and refractivepower values [Dpt.].

In another exemplary embodiment, the front lens-element group alsoconsists of four lens elements, wherein the first lens element and thesecond lens element have a negative refractive power, the third lenselement has a positive refractive power and the fourth lens element hasa negative refractive power. In this example, the third and fourth lenselements are combined to form a lens-element doublet having a negativetotal refractive power. The drawing in this respect in FIG. 5 togetherwith the associated table indicates a 1:2.0 24 mm lens with exact groupfocal lengths and refractive power values.

In a further lens, the front lens-element group consists of only onelens element having a negative or positive refractive power. Withrespect to this example, a 1:2.0 50 mm lens is indicated in FIG. 4 and a1:1.4 50 mm lens is indicated in FIG. 8 and more precise values areindicated by the respective tables.

In a further embodiment of the lens according to the invention, thefront lens-element group consists of two lens elements, wherein thefirst lens element has a positive refractive power and the second lenselement has a negative refractive power and both lens elements arecombined to form a lens-element doublet having a negative totalrefractive power. FIG. 9 of the drawing shows a corresponding exemplaryembodiment that is described in even more specific detail as a 1:1.4 50mm lens on the basis of the associated table.

In a further embodiment of the lens according to the invention, thefront lens-element group consists of three lens elements, wherein thefirst lens element and the second lens element have a negativerefractive power and the third lens element has a positive refractivepower. In this case, the second and third lens elements are combined toform a lens-element doublet having a negative total refractive power. Anexemplary embodiment of a 1:2.0 50 mm is described in greater detail inthe drawing with FIG. 3 together with the associated table.

In one advantageous configuration of a lens according to the embodimentsof the invention, the central group consists of one lens element havinga positive refractive power, wherein the iris diaphragm AP is arrangedin a stationary fashion upstream thereof. An exemplary embodiment of1:2.0 50 mm is described in greater detail in the drawing with FIG. 7together with the associated table.

In one development of the central group, the latter consists of two lenselements, wherein the first lens element has a negative refractive powerand the second lens element has a positive refractive power and the irisdiaphragm AP is arranged in a stationary fashion between the first andthe second lens elements. FIGS. 1 and 2 show such a central group.

In an alternatively optimized embodiment of the central group, thelatter consists of three lens elements, wherein the first lens elementhas a negative refractive power, the second lens element has a positiverefractive power and the third lens element has a positive refractivepower. In this case, the first and second lens elements are combined toform a lens-element doublet having a positive total refractive power,and, in an example illustrated in FIG. 5, the iris diaphragm AP isarranged in a stationary fashion between the lens-element doublet andthe third lens element.

In a further central group consisting of three lens elements, the firstlens element has a negative refractive power, the second lens elementhas a positive refractive power and the third lens element has anegative refractive power. The second and third lens elements arecombined to form a lens-element doublet having a positive totalrefractive power. In the example shown in FIG. 6, the iris diaphragm APis arranged in a stationary fashion upstream of the first lens element.

In an alternative embodiment, the central group consists of four lenselements, wherein the first lens element has a positive refractivepower, the second lens element has a negative refractive power and thethird and fourth lens elements have a positive refractive power. In thiscase, the second and third lens elements are combined to form alens-element doublet having a positive total refractive power, whereiniris diaphragm AP is arranged in a stationary fashion upstream of thefirst lens element. FIG. 3 shows such an exemplary embodiment.

FIG. 4 illustrates an exemplary embodiment comprising a central groupconsisting of five lens elements. The first lens element has a positiverefractive power and the second lens element has a negative refractivepower, and both lens elements are combined to form a lens-elementdoublet having a negative total refractive power. In this embodiment,the third lens element has a negative refractive power and the fourthlens element has a positive refractive power, wherein both lens elementsare combined to form a lens-element doublet having a positive totalrefractive power. The fifth lens element has a positive refractive powerand the iris diaphragm AP is arranged in a stationary fashion betweenthe two lens-element doublets.

In an alternative embodiment, the central group consists of six lenselements, wherein the first lens element has a positive refractive powerand the second lens element has a negative refractive power and bothlens elements are combined to form a first lens-element doublet having anegative total refractive power. The third lens element has a positiverefractive power, the fourth lens element has a negative refractivepower and the fifth lens element has a positive refractive power,wherein the fourth and fifth lens elements are combined to form a secondlens-element doublet having a negative total refractive power. The sixthlens element has a positive refractive power. In the example illustratedin FIGS. 8 and 9, the iris diaphragm AP is arranged in a stationaryfashion between the first lens-element doublet and the third lenselement.

In one particular embodiment of the lens according to the invention, theback lens-element group consists of three lens elements, wherein thefirst lens element has a positive refractive power and the second lenselement has a negative refractive power and both lens elements arecombined to form a lens-element doublet having a negative totalrefractive power. In an exemplary embodiment illustrated in FIGS. 8 and9, the third lens element has a negative refractive power.

In a further embodiment of the lens, the back lens-element grouplikewise consists of three lens elements, wherein the first lens elementhas a positive refractive power and the second lens element has anegative refractive power, and both lens elements are combined to form alens-element doublet having a positive total refractive power. In anexemplary embodiment shown in FIG. 3, the third lens element has apositive refractive power.

An alternative embodiment comprises a back lens-element group having twolens elements. The exemplary embodiments shown in FIGS. 1, 2 and 7 havein each case a positive refractive power in the first lens element and anegative refractive power in the second lens element.

A back lens-element group constructed from only one lens element havinga negative refractive power is illustrated by way of example in eachcase in FIGS. 4, 5 and 6.

Particularly advantageously, for the correction of monochromatic imagingaberrations, such as aperture aberrations, coma, astigmatism, curvatureand distortion, one or more lens elements having one or two asphericalsurfaces can be provided.

In the case of the exemplary embodiments specified in the drawing, saidaspherical surfaces are identified by an * in FIGS. 3, 4, 5, 6, 7, 8 and9.

In order to be able to ensure a short back focal distance (e.g. lessthan 25 mm, relative to the full-frame format) suitable for mirrorlessrecording systems and a position of the exit pupil that is suitable forimage recording sensors, the back lens-element groups can comprise atleast one lens element comprising an optical material having arefractive index ne of greater than 1.8. In this way, it is alsopossible to comply with limitations for the maximum diameters of thelens elements on account of limiting diameters, e.g. of a bayonet, thatare predefined by the camera or image recording system.

In all lens variants, the five lens-element groups described andillustrated individually represent a specifically necessary,self-contained constituent. Each of the lens-element groups can beoptically tuned by itself, which is expressed in particular by the ratioof focal length specified for each lens-element group to the total focallength.

Exemplary embodiments of the lenses according to the invention for thefull-frame format are illustrated schematically in the drawing and aredescribed in more specific detail below with reference to the figures.

In this case, in the figures:

FIG. 1 shows a lens-element section through a lens with the focal length90 mm and the maximum aperture number 2,

FIG. 2 shows a lens-element section through a lens with the focal length75 mm and the maximum aperture number 2,

FIGS. 3 and 5 show a lens-element section through in each case a lenswith the focal length 24 mm and the maximum aperture number 2,

FIG. 4 shows a lens-element section through a lens with the focal length50 mm and the maximum aperture number 2,

FIGS. 6 and 7 show a lens-element section through a lens with the focallength 90 mm and the maximum aperture number 2,

FIGS. 8 and 9 show a lens-element section through in each case a lenswith the focal length 50 mm and the maximum aperture number 1.4,

FIG. 10 show an image sensor in an imaging plane IM.

The movement paths of the respective lens-element groups during thefocusing process are illustrated below the lens-element sections in thefigures. Horizontal lines represent the positions of the lens-elementgroups G1, G2, G3, G4 and G5. The upper lines thereof identify thepositions in the focus setting infinity, the lower lines the positionsin the focus setting at the shortest object distance, and the centrallines the positions in a central focus setting. The perpendicular linesare assigned to the stationary lens-element groups G1, G3 and G5, andthe oblique lines to the displaceable focusing groups G2 and G4.

The lens-element sections in the drawing are illustrated to scale, suchthat relative indications, such as e.g. the material thickness in thecase of the lens element G5L2 in FIG. 1, which is considerably thinnerin the lens-element center in comparison with the material thickness atthe lens-element edge, can be diagrammatically established and checkedusing conventional geometric means. What is likewise disclosed in thisway is that in FIG. 1 the lens element G1L2 has in the lens-elementcenter a material thickness almost exactly two times (2.11 times)thicker than the lens element G1L1. These relationships are readilyapparent to the person skilled in the art and so lens-geometries canalso be inferred. By way of example, FIG. 1 illustrates that the firstlens-element group G1 as viewed from the object side is constructedfrom, in an order as viewed in the light direction to the imaging planeIM, two convexo-concave lens elements having a positive refractivepower, which are spaced apart by approximately the thickness of thefirst lens element G1L1 and are assigned downstream at a smallerdistance a cemented element consisting of a biconvex lens element havinga positive refractive power and a biconcave lens element having anegative refractive power.

Concrete exemplary embodiments are evident from the tables below forlenses with a focal length of 90 mm, 70 mm, 50 mm and 24 mm with amaximum aperture number of 2 and two lenses with a focal length of 50 mmand a maximum aperture number of 1.4, wherein the focal lengths are ineach case relative to the full-frame format (43.3 mm image circlediameter).

FIG. 1 Focal length f′ Refractive 2/90 [mm] power [Dpt.] f′Gr/f′Construction G1 176.5 5.7 2.0 + G2 55.0 18.2 0.6 + G3 −91.0 −11.0 −1.0 −G4 −90.4 −11.1 −1.0 − G5 72.1 13.9 0.8 +

FIG. 2 Focal length f′ Refractive 2/75 [mm] power [Dpt.] f′Gr/f′Construction G1 159.1 6.3 2.1 + G2 55.0 18.2 0.7 + G3 −91.0 −11.0 −1.2 −G4 −96.1 −10.4 −1.3 − G5 72.1 13.9 1.0 +

FIG. 3 Focal length f′ Refractive 2/24 [mm] power [Dpt.] f′Gr/f′Construction G1 −36.5 −27.4 −1.5 − G2 133.2 7.5 5.6 + G3 18.0 55.6 0.8 +G4 −24.9 −40.2 −1.0 − G5 125.6 8.0 5.2 +

FIG. 4 Focal length f′ Refractive 2/50 [mm] power [Dpt.] f′Gr/f′Construction G1 −83.9 −11.9 −1.7 − G2 63.0 15.9 1.3 + G3 29.9 33.5 0.6 +G4 −66.1 −15.1 −1.3 − G5 −84.8 −11.8 −1.7 −

FIG. 5 Focal length f′ Refractive 2/24 [mm] power [Dpt.] f′Gr/f′Construction G1 −37.6 −26.6 −1.6 − G2 105.1 9.5 4.4 + G3 18.4 54.3 0.8 +G4 −42.4 −23.6 −1.8 − G5 −499.2 −2.0 −20.8 −

FIG. 6 Focal length f′ Refractive 2/90 [mm] power [Dpt.] f′Gr/f′Construction G1 64.9 15.4 0.7 + G2 −61.3 −16.3 −0.7 − G3 32.4 30.9 0.4 +G4 −76.5 −13.1 −0.8 − G5 −111.2 −9.0 −1.2 −

FIG. 7 Focal length f′ Refractive 2/90 [mm] power [Dpt.] f′Gr/f′Construction G1 53.1 18.8 0.6 + G2 −36.4 −27.5 −0.4 − G3 78.8 12.7 0.9 +G4 79.8 12.5 0.9 + G5 −68.0 −14.7 −0.8 −

FIG. 8 Focal length f′ Refractive 1.4/50 [mm] power [Dpt.] f′Gr/f′Construction G1 80.9 12.4 1.6 + G2 −78.5 −12.7 −1.6 − G3 56.7 17.6 1.1 +G4 108.4 9.2 2.2 + G5 −87.9 −11.4 −1.8 −

FIG. 9 Focal length f′ Refractive 1.4/50 [mm] power [Dpt.] f′Gr/f′Construction G1 581.0 1.7 11.6 + G2 95.1 10.5 1.9 + G3 62.5 16.0 1.3 +G4 106.3 9.4 2.1 + G5 −104.3 −9.6 −2.1 −

1. A lens with a fixed focal length, comprising: five lens-elementgroups, wherein three lenses are mounted in a stationary manner and twolenses are mounted in a displaceable manner along an optical axis,wherein a first front lens-element group as viewed from an object sideis arranged in a stationary manner, a second lens-element groupconfigured as a focusing front group is arranged in a displaceablemanner, a third lens-element group having a positive or negativerefractive power and containing a stationary iris diaphragm, theaperture of which is adjustable, configured as a central group isarranged in a stationary manner, a fourth lens-element group configuredas a focusing back group is arranged in a displaceable manner, and afifth back lens-element group is arranged in a stationary mannerrelative to an imaging plane in a lens barrel and both the focusingfront group and the focusing back group are movable jointly relative toone another and to the lens-element groups arranged in a stationarymanner in order to focus the lens on objects at different objectdistances.
 2. The lens as claimed in claim 1, wherein the frontlens-element group and the back lens-element group have a negativerefractive power, or the front lens-element group and the backlens-element group have a positive refractive power, or the frontlens-element group has a positive refractive power and the backlens-element group has a negative refractive power, or the frontlens-element group has a negative refractive power and the backlens-element group has a positive refractive power.
 3. The lens asclaimed in claim 2, wherein the focusing front group and the focusingback group have a positive refractive power, or the focusing front grouphas a positive refractive power and the focusing back group has anegative refractive power, or the focusing front group and the focusingback group have a negative refractive power, or the focusing front grouphas a negative refractive power and the focusing back group has apositive refractive power.
 4. The lens as claimed in claim 3, whereinboth the focusing front group and the focusing back group move away fromthe imaging plane during focusing from infinity to the near setting. 5.The lens as claimed in claim 3, wherein the focusing front group movesaway from the imaging plane during focusing from infinity to the nearsetting and in that the focusing back group moves toward the imagingplane during focusing from infinity to the near setting.
 6. The lens asclaimed in claim 3, wherein both the focusing front group and thefocusing back group move toward the imaging plane during focusing frominfinity to the near setting.
 7. The lens as claimed in claim 3, whereinthe focusing front group moves toward the imaging plane during focusingfrom infinity to the near setting and in that the focusing back groupmoves away from the imaging plane during focusing from infinity to thenear setting.
 8. The lens as claimed in claim 7, wherein the ratio f1/fof the focal lengths f1 of the front lens-element group and f of theentire lens lies between −40 and 30, the ratio f2/f of the focal lengthsf2 of the focusing front group and f of the entire lens lies between −10and 20, the ratio f3/f of the focal lengths f3 of the central group andf of the entire lens lies between −30 and 40, the ratio f4/f of thefocal lengths f4 of the focusing back group and f of the entire lenslies between −10 and 20, and the ratio f5/f of the focal lengths f5 ofthe back lens-element group and f of the entire lens lies between −40and
 30. 9. The lens as claimed in claim 8, wherein the lens has a ratioof the total focal length f to the image circle diameter in the imagingplane of between 0.3 and
 5. 10. The lens as claimed in claim 9, whereinthe ratio V of the volume of the respective focusing elements to theimage circle diameter in the imaging plane raised to the third powerlies below 0.1 (V/Bd3<0.1), in particular below 0.08, or the weight ofthe respective focusing group lies below 15 g.
 11. The lens as claimedin claim 10, wherein either the front lens-element group consists offour lens elements, wherein the first lens element, the second lenselement and the third lens element have a positive refractive power andthe fourth lens element has a negative refractive power, wherein thethird lens element and the fourth lens element are combined to form alens-element doublet having a negative total refractive power, or thefront lens-element group consists of four lens elements, wherein thefirst lens element and the second lens element have a positiverefractive power, and the third lens element has a negative refractivepower and the fourth lens element has a positive refractive power,wherein either the third lens element and the fourth lens element arecombined to form a lens-element doublet having a negative totalrefractive power, or the second lens element, the third lens element andthe fourth lens element are combined to form a cemented element having apositive total refractive power, or the front lens-element groupconsists of four lens elements, wherein the first lens element and thesecond lens element have a negative refractive power and the third lenselement has a negative refractive power, and the fourth lens element hasa negative refractive power, wherein the third lens element and thefourth lens element are combined to form a lens-element doublet having anegative total refractive power or the front lens-element group consistsof one lens element having a negative or positive refractive power, orthe front lens-element group consists of two lens elements, wherein thefirst lens element has a positive refractive power and the second lenselement has a negative refractive power, which lens elements arecombined to form a lens-element doublet having a negative totalrefractive power, or the front lens-element group consists of three lenselements, wherein the first lens element and the second lens elementhave a negative refractive power and the third lens element has apositive refractive power, wherein the second lens element and the thirdlens element are combined to form a lens-element doublet having anegative total refractive power.
 12. The lens as claimed in claim 11,wherein either the central group consists of one lens element having apositive refractive power and the iris diaphragm is arranged in astationary fashion upstream thereof, or the central group consists oftwo lens elements, wherein the first lens element has a negativerefractive power and the second lens element has a positive refractivepower, and the iris diaphragm is arranged in a stationary fashionbetween the first lens element and the second lens element, or thecentral group consists of three lens elements wherein the first lenselement has a negative refractive power, the second lens element has apositive refractive power and the third lens element has a positiverefractive power, wherein the first lens element and the second lenselement are combined to form a lens-element doublet having a positivetotal refractive power, and the iris diaphragm is arranged in astationary fashion between the lens-element doublet and the third lenselement, or the central group consists of three lens elements, whereinthe first lens element has a negative refractive power, the second lenselement has a positive refractive power and the third lens element has anegative refractive power, wherein the second and third lens elementsare combined to form a lens-element doublet having a positive totalrefractive power, and the iris diaphragm is arranged in a stationaryfashion upstream of the first lens element, or the central groupconsists of four lens elements, wherein the first lens element has apositive refractive power, the second lens element has a negativerefractive power, the third lens element has a positive refractive powerand the fourth lens element has a positive refractive power, and thesecond and third lens elements are combined to form a lens-elementdoublet having a positive total refractive power, and the iris diaphragmis arranged in a stationary fashion upstream of the first lens element,or the central group consists of five lens elements wherein the firstlens element has a positive refractive power and the second lens elementhas a negative refractive power and both lens elements are combined toform a lens-element doublet having a negative total refractive power,the third lens element has a negative refractive power and the fourthlens element has a positive refractive power and both lens elements arecombined to form a lens-element doublet having a positive totalrefractive power, and the fifth lens element has a positive refractivepower, and the iris diaphragm is arranged in a stationary fashionbetween the two lens-element doublets, or the central group consists ofsix lens elements, wherein the first lens element has a positiverefractive power and the second lens element has a negative refractivepower and both lens elements are combined to form a lens-element doublethaving a negative total refractive power, the third lens element has apositive refractive power, the fourth lens element has a negativerefractive power and the fifth lens element has a positive refractivepower and the fourth and fifth lens elements are combined to form alens-element doublet having a negative total refractive power, and thesixth lens element has a positive refractive power, and the irisdiaphragm is arranged in a stationary fashion between the firstlens-element doublet and the third lens element.
 13. The lens as claimedin claim 12, wherein either the back lens-element group consists ofthree lens elements, wherein the first lens element has a positiverefractive power, the second lens element has a negative refractivepower and both lens elements are combined to form a lens-element doublethaving a negative total refractive power, and the third lens element hasa negative refractive power, or the back lens-element group consists ofthree lens elements, wherein the first lens element has a positiverefractive power, the second lens element has a negative refractivepower and both lens elements are combined to form a lens-element doublethaving a positive total refractive power, and the third lens element hasa positive refractive power, or the back lens-element group consists oftwo lens elements, wherein the first lens element has a positiverefractive power and the second lens element has a negative refractivepower, or the back lens-element group consists of one lens elementhaving a negative refractive power.